1. Field of the Invention
The invention relates to an electric propulsion pod for a ship, which pod has an electric motor fitted into a design of propulsion pod which ensures favorable flow of the water around the pod, the propulsion pod being located on the bottom of the ship by means of a hollow -access shaft and the heat generated by the electric motor being rejected via the surface of the propulsion pod to the water flowing around it.
2. Description of the Prior Art
An electric propulsion pod for a ship corresponding to the above can be seen in the applicant""s publication with the title: Siemens-Schottel-Propulsor (SSP) xe2x80x9cThe Podded Electric Drive with Permanently Excited Motorxe2x80x9d, presented to the AES 97xe2x80x94All Electric Ship 13-14.03.97, Paris. The publication on the Siemens-Schottel-Propulsor (SSP) shows an electric propulsion pod for a ship with a motor surface-cooled which is in a simple manner and is in the form of a permanently excited synchronous motor.
This motor, whose more precise details can be seen from FIG. 2 of the publication, is completely encapsulated and maintenance-free.
The object of the invention is to provide a solution which permits reliable cooling of the motor even when the electric propulsion pod is employed at overload in tropical waters with high water temperatures. In addition, the working temperature of the electric motor is to be lowered and a more uniform temperature of the individual components of the motor, for example the coil winding heads, is to be achieved.
The object is, in principle, achieved in that the heat is rejected, by heat rejection areas in the water both from the propulsion pod (17, 23) and from the access shaft (16, 18), that are employed to improve the heat conduction and rejection. The inclusion of the access shaft in the heat rejection from the motor very advantageously achieves the effect that the cooling of the motor is not limited to the surface of the propulsion pod only. This can advantageously occur, in accordance with the invention, without departure from the simple surface cooling as the cooling principle.
An embodiment of the invention provides for improving the heat rejection to be an increase in the effective heat rejection area. In the electric propulsion pod known from the prior art, only the outer wall of the propulsion pod, in the winding region of the electric motor, is provided as the effective heat rejection area. In this case direct heat rejection from the shrunk-in inner part takes place at the outer wall. This effective heat rejection area is substantially increased according to the present invention. The result is an advantageously improved thermal behavior of the propulsion pod.
A further embodiment of the invention provides for improving the heat rejection to be an increase in the temperature of the effective heat rejection area. Increasing the temperature of the effective heat rejection area advantageously increases the temperature difference relative to the surrounding sea water and satisfactory cooling of the propulsion system is ensured even in the case where the propulsion system is employed in tropical waters with water temperatures of between 30xc2x0 C. and 35xc2x0 C. This is important, particularly for cruise ships which pass through the Red Sea, for example.
In order to improve the heat conduction, in particular to improve the heat conduction from the winding region of the electric motor, provision is made for a material with higher thermal conductivity than steel to be used. For this purpose, a material made of non-ferrous alloy with good thermal conductivity is particularly advantageous. Copper-containing non-errous alloys, in particular, have a higher thermal conductivity than steel. When special copper bronzes are used, there is a further essential advantage that no growth occurs on the surface. It is therefore possible to dispense with the use of an anti-fouling paint on the surface of the pod and that of the transition between the pod and the access shaft. Dispensing with a coat of paint, in this way, leads to a not insubstantial increase in the surface temperature of the heat rejection area because anti-fouling paint coats have a thermal conductivity which is lower than that of metal by a power of 10. They act as an insulating layer and impair the heat rejection. An unexpected advantage is achieved by the use of a special copper bronze, the so-called propeller bronze G-CU Al 10 Ni being recommended in this case; not only is the thermal conductivity improved because such materials are better heat conductors than steel but a substantially increased heat rejection temperature is also achieved.
Provision is also made in the electric propulsion pod for the propulsion pod to have a reduced wall thickness, in the part directed into the access shaft, as compared with the wall thickness present in the part with the favorable flow configuration. This advantageously provides particularly good heat rejection into the access shaft from the part of the pod surface not directly cooled by the sea water. The wall thickness in the part of the propulsion pod directed into the shaft can be reduced as much as is permitted by the casting technique. There is, therefore, an essentially higher surface temperature in this region as compared with the rest of the central region of the motor pod, which has to have a favorable flow configuration and therefore has a relatively large wall thickness in the center.
Provision is also made for the propulsion pod to have an enlarged surface on the part directed into the access shaft, an enlarged surface due to ribs, beads or honeycomb sheet, for example. This advantageously achieves the effect that the heat rejecting surface is essentially enlarged so that, in this region, increased heat rejection can occur. The heat rejected is convectively distributed by the air located in the hollow access shaft and, in this way, passes via the large surface of the access shaft into the sea water.
An embodiment of the invention provides for components of the enlarged surface to have heat conduction devices (heat ducts) which are in connection with the inside of the electric motor. In this way, the surface temperature of the enlarged surface can be raised and, therefore, the heat rejection to the air circulating within the access shaft can be still further increased. This does not depart from the simple cooling which is a principle of the invention.
Additionally, or as an alternative, the access shaft can have a lower part which has, at least in part, a double-walled configuration, the inside of the double-walled part having heat conducting media such as air or water. Devices, for example fans, are also, if appropriate, provided in the access shaft for circulating the access shaft air, these devices being used to maintain a stable circulation. By this way, the heat rejected by the pod into the access shaft can be satisfactorily rejected to the access shaft wall and be led along the latter and through it to the sea water in a satisfactory manner.
The above devices are advantageously located in the lower part only of the access shaft, around which sea water always washes. The transition between the access shaft and the ship is located above the water line and thus sea water only washes around parts of the upper part of the access shaft. Reliable heat removal is achieved by the arrangement of the devices for increasing the heat removal in the lower part of the access shaft. If the propulsion pod is arranged on a short access shaft whose transition to the ship occurs below the water line, for which provision is likewise made, the corresponding devices are of course located in the whole of the access shaft. Because, in principle, no provision is made for the arrangement of the transition between the access shaft and the ship at the water line, only these two alternatives need to be considered for the arrangement of the heat rejection components in the access shaft.
As a supplement, or likewise as an alternative, provision is made for the propulsion pod to have devices which contain heat transfer media (heat ducts); such that, the heat can be advantageously led away directly, so that a particularly effective, low-cost and simple solution results. Pursuing this principle, furthermore, provision is made for the electric motor to have a hollow shaft, which is open at both ends, through which sea water can flow and which has, if required, a conical configuration. In consequence, cooling of the electric motor also takes place from the inside.
In another embodiment, provision is made for a convective cooling circuit to be arranged in the shaft of the electric motor, which circuit transports heat from the center of the electric motor to the cool ends. Thus, the area of the hub and, in fact, a part of the propeller surface can be advantageously used for conducting away the heat.
Provision is also made for the coil winding heads of the electric motor to have convectively operating heat ducts, which lead to the cool outer ends, to side fins or into the lower part of the access shaft. The coil winding heads are not in direct contact with the outer wall of the propulsion pod but they develop a substantial quantity of heat because of the currents flowing within them. In some cases, therefore, additional cooling of the coil winding heads is necessary and this can take place in a particularly simple manner by the heat ducts described above. The surface of the cool outer ends, the side fins or the lower part of the access shaft is utilized particularly favorably for this purpose.
In order to conduct away the heat developed by the coil winding heads directly the latter are also advantageously provided with heat bridges to the outer wall of the propulsion pod. In the case of smaller propulsion systems, it is then even possible to dispense with heat ducts and further cooling components. The cooling of the propulsion pod at the outer wall is then sufficient.
These heat conducting bridges advantageously consist of heat conducting plastic with a filler material of a material which conducts heat particularly well. Epoxy resin can, for example, be considered as the plastic, while minerals can be used as the filler material. In this arrangement, the heat conducting bridges can be larger than the coil winding head dimensions and can, for example, be configured as heat conducting rings, which advantageously have parting lines between the individual coil winding head sections. The result is a particularly large-volume configuration of the heat conducting bridges with good thermal conduction from the coil winding heads to the outer wall of the propulsion pod.
Provision is also made for the propulsion pod and/or the lower part of the access shaft to have surface enlarging elements, for example external ribs or external beads, to improve the cooling. This likewise achieves improved heat removal from the motor into the sea water, it being possible, in a particularly advantageous manner, for these external ribs or external beads also to undertake flow guidance functions which support the action of fins.
Cooling ducts, through which water flows and which have a conical configuration to avoid blocking due to flotsam, are likewise provided at the transition between the access shaft and the propulsion pod; this provides particularly good cooling for this region. Heat ducts, which are led out from the inside of the propulsion motor, can advantageously end at the cooling ducts.
Provision is also made, if appropriate, for the external region of the motor and/or the motor/access shaft transition region to have an at least partially double-walled configuration, the space between the two walls being configured so that a coolant, in particular water, can flow through it. In such double-walled spaces, a circulation occurs due to the one-sided supply of heat so that these double-walled regions can be used as good heat rejection regions. In addition, they have the advantage that they can, for example, reinforce the lower part of the access shaft or that they can contribute to the formation of a shape which is particularly favorable to flow. This therefore makes it possible to achieve a combined effect.
The heat conducting devices, like the electric motor, are of maintenance-free design. This is readily possible because they operate without circulating pumps. They can therefore be configured as a unit forming a block with the propulsion pod motor, for which no provision is made for maintenance, or even for repair, in service operation. Because the heat conducting and rejection devices are completely located in the lower part of the access shaft, they do not interfere with the dismantling of the lower part of the access shaft. This dismantling work takes place by divers when the propulsion pod is exchanged for repair, the ship remaining in the water. As compared with the known heat-exchanger solutions with heat exchangers in the ship or on deck, there are therefore substantial handling and cost advantages.